Implantable stimulation devices generate and deliver electrical stimuli to nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, occipital nerve stimulators to treat migraine headaches, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. Implantable stimulation devices may comprise a microstimulator device of the type disclosed in U.S. Patent Application Publication 2008/0097529, or a spinal cord stimulator of the type disclosed in U.S. Patent Application Publication 2007/0135868, or other forms.
Microstimulator devices typically comprise a small, generally-cylindrical housing which carries electrodes for producing a desired electric stimulation current. Devices of this type are implanted proximate to the target tissue to allow the stimulation current to stimulate the target tissue to provide therapy. A microstimulator's case is usually on the order of a few millimeters in diameter by several millimeters to a few centimeters in length, and usually includes or carries stimulating electrodes intended to contact the patient's tissue. However, a microstimulator may also or instead have electrodes coupled to the body of the device via a lead or leads.
Some microstimulators 2 in the prior art contain only one or two electrodes, such as is shown in FIG. 1, and are thus referred to as “bi-electrode” microstimulators. An example of a bi-electrode microstimulator device includes the Bion® device made by Boston Scientific Neuromodulation Corporation of Valencia, Calif. A single anode electrode, Eanode, sources current into a resistance R, i.e., the user's tissue. The return path for the current is provided by a single cathode electrode, Ecathode. Either of the anode or cathode electrodes could comprise the case of the device, or other conductive part of the case. Current flows by operation of a current source 20, which typically comprises a Digital-to-Analog Converter, or “DAC” 20, which is programmable to provide a desired therapeutic current, Iout, to the patient's tissue R. Such current Iout is typically pulsed as shown in the bottom of FIG. 1, and can have a frequency and duty cycle suitable for the patient.
A current source or DAC could also be coupled to the anode. However, as shown, the anode is coupled to a compliance voltage, V+, of sufficient strength to provide the current, Iout, programmed into the DAC 20. This compliance voltage can be generated from a battery voltage, Vbat, provided by a battery 12 in the microstimulator 2. A DC-DC converter 22 is used to boost Vbat to the desired compliance voltage V+, and is controlled by a V+ monitor and adjust circuitry 18. Because such circuitry for compliance voltage generation is well known, and not directly germane to the issues presented by this disclosure, further elaboration is not provided.
Also shown in FIG. 1 is the provision of decoupling or blocking capacitors 42 and 44 hardwired to the anode and cathode respectively. As is well known, such decoupling capacitors only allow the passage of AC components of the current provided by the DAC 20, and thus prevent the DC injection of current into the patient's tissue R (Idc=0). Preventing DC current injection into the tissue is desired for safety: when the DC component of the current is removed, the possibility of current building up in the patient's tissue is minimized.
Although two decoupling capacitors 42 and 44 are shown in FIG. 1, only one is needed to prevent DC current injection, which one capacitor is coupled to the DAC 20. Thus, when the DAC 20 appears on the cathode side of the current path, only a cathode capacitor 44 is needed, as shown in FIG. 2. Likewise, were the DAC 20 on the anode side of the current path, only an anode capacitor 42 would be needed (not shown in FIG. 2). Using only one decoupling capacitor 42 or 44 is preferred because the decoupling capacitors tend to be rather large in comparison to the rest of the circuitry within the microstimulator 2, and hence take up significant room in the case. Reducing the number of decoupling capacitors therefore allows the microstimulator 2 to be made smaller, which simplifies the implanting procedure and conveniences the patient.
Bi-electrode microstimulators 2 benefit from simplicity. Because of their small size, such microstimulators 2 can be implanted at site requiring patient therapy, and without leads to carry the therapeutic current away from the body as mentioned previously. However, such bi-electrode microstimulators lack therapeutic flexibility: once implanted, the single cathode/anode combination will only recruit nerves in their immediate proximity, which generally cannot be changed unless the position of the device is manipulated in a patient's tissue.
To improve therapeutic flexibility, microstimulators having more than two electrodes have been proposed, and such devices are referred to herein as “multi-electrode” microstimulators to differentiate them from bi-electrode microstimulators discussed above. When increasing the number of electrodes in this fashion, the electrodes can be selectively activated once the device is implanted, providing the opportunity to manipulate therapy without having to manipulate the position of the device.
Exemplary multi-electrode microstimulators 4, 6, and 8 are shown in FIGS. 3A-3C respectively, and are disclosed in the '529 Publication referenced above. As its name suggests, the multi-electrode microstimulator comprises a plurality of electrodes, which electrodes may be located on the case in various manners, such as on two sides of the case as shown in the pictures at the bottom right of FIGS. 3A-3C. In this and subsequent examples, it should be noted that any of the electrodes can comprise the implant's case, or conductive portions thereof.
In the embodiment of FIG. 3A, there is provided a dedicated anode electrode, Eanode. By contrast, one of E1 cathode-Encathode is selectable as the cathode via cathode switches 621-62g. Selecting a particular cathode by closing its corresponding cathode switch couples that cathode to the DAC 20. For example, FIG. 3A shows the circuit that is completed when E1 cathode is selected. Notice that this design employs a single decoupling capacitor 42 in the anode path.
Also shown in FIG. 3A are recovery switches 64 and 661-66n. As explained in the above-referenced '529 Publication, the recovery switches 64 and 661-66n are activated at some point after provision of a stimulation pulse, and have the goal of recovering any remaining charge left on the decoupling capacitor 44 and in the patient's tissue. Thus, after a stimulation pulse, the recovery switch 64 and at least one of switches 661-66n are closed. Closure of these switches places the same reference voltage on each plate of the decoupling capacitor 302, thus removing any stored charge. In one embodiment, for convenience, the reference voltage used is the battery voltage, Vbat, of the battery in the microstimulator 4, although any other reference potential could be used. Thus, during recovery, Vbat is placed on the left plate of capacitor 44 via recovery switch 64, and is likewise placed on the right plate (through the patient's tissue, R) via one or all of the recovery switches 661-66n. As recovery is discussed in further detail in the '529 Publication, and it is not directly germane to this disclosure, it is not further discussed.
The embodiment of FIG. 3B improves upon the embodiment of FIG. 3A in that it allows the anode electrode to be selected as well as the cathode electrode. Thus, the device contains N electrodes, E1-En, any of which can comprise the anode or cathode at any given time. As before, which electrode acts as the cathode is determined by selecting a particular cathode switch 621-62n. Which electrode acts as the anode is determined by selecting a particular anode switch 681-68n. For example, FIG. 3B shows the circuit that is completed when E1 is selected as the anode, and E2 is selected as the cathode. Notice again that this design employs a single decoupling capacitor 42 in the anode path, regardless of which electrode is selected as the anode.
The embodiments of FIGS. 3A and 3B are similar in that the singular decoupling capacitor 42 prevents DC current injection to the patient's tissue R, i.e., Idc=0. As a result, these designs can be regarded as generally safe for the reasons stated earlier. Moreover, these designs are generally compact: most significantly, they only require a single decoupling capacitor 42.
However, the designs of FIGS. 3A and 3B have a shortcoming arising from their provision of a single DAC 20, namely the inability to simultaneously and independently modify the current at two or more different cathodes. Being able to so modify the current at two (or more) different cathode electrodes is desired in one example to “steer” current from one cathode to another. The concept of current steering is addressed in U.S. Patent Application Publication 2007/0239228, and so is only briefly explained here with reference to FIG. 4. FIG. 4 presents an initial condition, in which E2 has been designated as the anode, and E4 has been designated as the cathode. As the net amount of current provided by these electrodes must equal zero, E2 sources 10 mA, while E4 sinks −10 mA. In the next condition, some of the sink current (−2 mA) has been moved or “steered” from cathode electrode E4 to E3. Steering in 2 mA increments continues until in the last condition, all of the sink current (−10 mA) has been moved to cathode E3, while original cathode E4 is now off. Anode current can be similarly steered in some stimulators, but this is not shown. Being able to steer the current in this fashion not only improves the complexity of therapy that can be provided to the patient, but also allows for safe and comfortable experimentation during fitting to determine the best electrodes to activate for a particular patient. However, the designs of FIGS. 3A and 3B cannot so steer the current at two different cathodes simultaneously.
An embodiment disclosed in the above-referenced '529 Publication capable of current steering is shown in FIG. 3C. This microstimulator 8 improves from the microstimulator 6 of FIG. 3B in that each electrode E1-En has its own dedicated, and independently-controllable, DAC 201-20n. As a result, more than one electrode can be selected as the cathode at any given time via selection of two or more of the cathode selection switches 621-62n, and the current sunk at each can be independently controlled by the corresponding DACs 201-20n, which enables current steering of the sort depicted in FIG. 4.
Unfortunately, microstimulator 8 of FIG. 3C has a shortcoming related to its provision of a single decoupling capacitor 42, namely the possibility of direct DC current injection into the patient's tissue R during current steering. This is illustrated in FIG. 5. The first circuit shows the selection of Ex as the anode, and only a single electrode Ey as the cathode. In this condition, the decoupling capacitor 42 prevents DC current injection through the entirety of the current path. However, the second circuit shows the selection of electrodes Ey and Ez as cathodes, such as might occur when some of the current at Ey is steered to Ez. In this configuration, the decoupling capacitor 42 prevents DC current injection in the anode path Idca=0. However, no such decoupling capacitor appears in the cathode paths, and therefore DACs 20y and 20z are not prevented from providing a DC current through the patient's tissue. In short, while the design of FIG. 3C allows for current steering, and might be relatively compact by virtue of its single capacitor 42, it does not guarantee an absence of direct DC current injection into each cathode electrode.
FIG. 6 provides yet another design for a multi-electrode implantable stimulator 10. This type of design is often used in a spinal cord stimulator (SCS), such as that illustrated in the above-referenced '868 application. An SCS 10 will typically have a case which is coupled by leads to an electrode array. The electrode array is implanted into the patient's spine, while the case is implanted at a distant, less-critical location, such as in the patient's buttocks. Because the case is not implanted right at the location requiring stimulation, the case of the SCS 10 can typically be larger than the various microstimulators illustrated to this point.
As seen in FIG. 6, the SCS 10 has a plurality of electrodes E1-En. Hardwired to each electrode are decoupling capacitors C1-Cn, and coupled to each of these capacitors are DACs 201-20n. In this particular design, the DACs can be controlled to operate as either current sources or current sinks, and thus their associated electrodes can comprise anodes or cathodes. Shown in FIG. 6 is an example in which DAC 202 is active as a source thus designating E2 as an anode, and DAC 204 is active as a sink thus designating E4 as a cathode. All other DACs, and their associated electrodes, are inactive.
Because the SCS 10 has individually-controllable DACs dedicated to each of the electrodes, current can readily be steered between the two electrodes. That is, two or more of the electrodes can act as cathodes (sinks) and/or two or more of the electrodes can act as anodes (sources) at one time. Moreover, because each electrode is hardwired to a decoupling capacitor C1-Cn, there is no risk of direct DC current injection into the tissue R of the patient, even during current steering.
The SCS 10 system therefore has many favorable functional benefits. However, the requirement that each of the N electrodes be hardwired to a dedicated decoupling capacitor means that N decoupling capacitors must be provided. As mentioned before, these capacitors can take up significant space in the case of the implantable stimulator. This may not be as critical of a concern where the implantable stimulator is an SCS 10 for example, because as mentioned, that type of device can generally support a larger case. However, where a small-sized microstimulator is concerned, the requirement of N capacitors for each of the N electrodes is prohibitive.
Accordingly, the inventor believes that the implantable stimulator art, and particularly the multi-electrode microstimulator art, would benefit from an architecture that would minimize device size and ensure patient safety. Specifically desirable would be a design that would minimize the number of decoupling capacitors required, but which would still prevent DC current injection even during current steering. Embodiments of such a solution are provided herein.